T-cell development depends upon interactions between thymocytes and thymic epithelial cells (TECs). The engagement of delta-like 4 (DL4) on TECs by Notch1 expressed by blood-borne BM-derived precursors is essential for T-cell commitment in the adult thymus. In contrast to the adult, the earliest T-cell progenitors in the embryo originate in the fetal liver and migrate to the nonvascularized fetal thymus via chemokine signals. Within the fetal thymus, some T-cell precursors undergo programmed TCR gamma and TCR delta rearrangement and selection, giving rise to unique gamma delta T cells. Despite these fundamental differences between fetal and adult T-cell lymphopoiesis, we show here that DL4-mediated Notch signaling is essential for the development of both alpha beta and gamma delta T-cell lineages in the embryo. Deletion of the DL4 gene in fetal TECs results in an early block in alpha beta T-cell development and a dramatic reduction of all gamma delta T-cell subsets in the fetal thymus. In contrast to the adult, no dramatic deviation of T-cell precursors to alternative fates was observed in the fetal thymus in the absence of Notch signaling. Taken together, our data reveal a common requirement for DL4-mediated Notch signaling in fetal and adult thymopoiesis.